<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>作用域 | Guru Note</title>
    <meta name="generator" content="VuePress 1.7.1">
    <link rel="stylesheet" href="./font/index.css">
    <script src="/docs/icon/svg.js"></script>
    <meta name="description" content="Sea of dawn">
    
    <link rel="preload" href="/docs/assets/css/0.styles.efa081cd.css" as="style"><link rel="preload" href="/docs/assets/js/app.cdec4db9.js" as="script"><link rel="preload" href="/docs/assets/js/2.993bd611.js" as="script"><link rel="preload" href="/docs/assets/js/72.8665d6b8.js" as="script"><link rel="preload" href="/docs/assets/js/7.5a12abef.js" as="script"><link rel="prefetch" href="/docs/assets/js/10.c3177bdb.js"><link rel="prefetch" href="/docs/assets/js/100.60c23478.js"><link rel="prefetch" href="/docs/assets/js/101.7ea29e1b.js"><link rel="prefetch" href="/docs/assets/js/102.7f47e485.js"><link rel="prefetch" href="/docs/assets/js/103.ea9ce400.js"><link rel="prefetch" href="/docs/assets/js/104.4ec6a518.js"><link rel="prefetch" href="/docs/assets/js/105.75526347.js"><link rel="prefetch" href="/docs/assets/js/106.01f6b03b.js"><link rel="prefetch" href="/docs/assets/js/107.69fe0811.js"><link rel="prefetch" href="/docs/assets/js/108.fa23768b.js"><link rel="prefetch" href="/docs/assets/js/109.90df1698.js"><link rel="prefetch" href="/docs/assets/js/11.54347528.js"><link rel="prefetch" href="/docs/assets/js/110.dd8d7227.js"><link rel="prefetch" href="/docs/assets/js/111.ccf25ceb.js"><link rel="prefetch" href="/docs/assets/js/112.6963298f.js"><link rel="prefetch" href="/docs/assets/js/113.30ceb3d8.js"><link rel="prefetch" href="/docs/assets/js/114.42ef6603.js"><link rel="prefetch" href="/docs/assets/js/115.f1db7817.js"><link rel="prefetch" href="/docs/assets/js/116.92971223.js"><link rel="prefetch" href="/docs/assets/js/117.c5a65e7e.js"><link rel="prefetch" href="/docs/assets/js/118.e329035d.js"><link rel="prefetch" href="/docs/assets/js/119.cbb17c5e.js"><link rel="prefetch" href="/docs/assets/js/12.c8144ee8.js"><link rel="prefetch" href="/docs/assets/js/120.8537f6a6.js"><link rel="prefetch" href="/docs/assets/js/121.257d3851.js"><link rel="prefetch" href="/docs/assets/js/122.96a5f921.js"><link rel="prefetch" href="/docs/assets/js/123.2220fd12.js"><link rel="prefetch" href="/docs/assets/js/124.552b1a29.js"><link rel="prefetch" href="/docs/assets/js/125.4e36fe37.js"><link rel="prefetch" href="/docs/assets/js/126.3939cdcc.js"><link rel="prefetch" href="/docs/assets/js/127.014df434.js"><link rel="prefetch" href="/docs/assets/js/13.edc237a8.js"><link rel="prefetch" href="/docs/assets/js/14.66270d4f.js"><link rel="prefetch" href="/docs/assets/js/15.0804164b.js"><link rel="prefetch" href="/docs/assets/js/16.dec928ab.js"><link rel="prefetch" href="/docs/assets/js/17.bbfc90a8.js"><link rel="prefetch" href="/docs/assets/js/18.76904860.js"><link rel="prefetch" href="/docs/assets/js/19.80a98011.js"><link rel="prefetch" href="/docs/assets/js/20.c39e42bd.js"><link rel="prefetch" href="/docs/assets/js/21.287b744a.js"><link rel="prefetch" href="/docs/assets/js/22.c9506be7.js"><link rel="prefetch" href="/docs/assets/js/23.9732a229.js"><link rel="prefetch" href="/docs/assets/js/24.21ed1f05.js"><link rel="prefetch" href="/docs/assets/js/25.8f1926f6.js"><link rel="prefetch" href="/docs/assets/js/26.61b6eb9f.js"><link rel="prefetch" href="/docs/assets/js/27.3706753c.js"><link rel="prefetch" href="/docs/assets/js/28.b6b182cf.js"><link rel="prefetch" href="/docs/assets/js/29.ae979ad9.js"><link rel="prefetch" href="/docs/assets/js/3.a7f8dd77.js"><link rel="prefetch" href="/docs/assets/js/30.f2233269.js"><link rel="prefetch" href="/docs/assets/js/31.06780314.js"><link rel="prefetch" href="/docs/assets/js/32.2dad91d1.js"><link rel="prefetch" href="/docs/assets/js/33.87ec6e22.js"><link rel="prefetch" href="/docs/assets/js/34.7b75f220.js"><link rel="prefetch" href="/docs/assets/js/35.3184991f.js"><link rel="prefetch" href="/docs/assets/js/36.28248fbe.js"><link rel="prefetch" href="/docs/assets/js/37.5824a979.js"><link rel="prefetch" href="/docs/assets/js/38.99a364fe.js"><link rel="prefetch" href="/docs/assets/js/39.a5f43732.js"><link rel="prefetch" href="/docs/assets/js/4.a55d89ed.js"><link rel="prefetch" href="/docs/assets/js/40.629f78f7.js"><link rel="prefetch" href="/docs/assets/js/41.e4f58d1c.js"><link rel="prefetch" href="/docs/assets/js/42.9f41aa47.js"><link rel="prefetch" href="/docs/assets/js/43.636412b6.js"><link rel="prefetch" href="/docs/assets/js/44.3b491aef.js"><link rel="prefetch" href="/docs/assets/js/45.77df19bc.js"><link rel="prefetch" href="/docs/assets/js/46.01f53ddd.js"><link rel="prefetch" href="/docs/assets/js/47.476e85c9.js"><link rel="prefetch" href="/docs/assets/js/48.198502dc.js"><link rel="prefetch" href="/docs/assets/js/49.0d59d332.js"><link rel="prefetch" href="/docs/assets/js/5.1597c0f8.js"><link rel="prefetch" href="/docs/assets/js/50.02baf101.js"><link rel="prefetch" href="/docs/assets/js/51.c9fbd54d.js"><link rel="prefetch" href="/docs/assets/js/52.4cb4459b.js"><link rel="prefetch" href="/docs/assets/js/53.248450d7.js"><link rel="prefetch" href="/docs/assets/js/54.83f12d1e.js"><link rel="prefetch" href="/docs/assets/js/55.b18accba.js"><link rel="prefetch" href="/docs/assets/js/56.8a160b09.js"><link rel="prefetch" href="/docs/assets/js/57.b854a940.js"><link rel="prefetch" href="/docs/assets/js/58.4500f315.js"><link rel="prefetch" href="/docs/assets/js/59.59400e36.js"><link rel="prefetch" href="/docs/assets/js/6.887ba020.js"><link rel="prefetch" href="/docs/assets/js/60.4dd5b5bb.js"><link rel="prefetch" href="/docs/assets/js/61.50d8c8f6.js"><link rel="prefetch" href="/docs/assets/js/62.17d10daa.js"><link rel="prefetch" href="/docs/assets/js/63.d5f821cc.js"><link rel="prefetch" href="/docs/assets/js/64.7bf2519f.js"><link rel="prefetch" href="/docs/assets/js/65.0a1c9bc8.js"><link rel="prefetch" href="/docs/assets/js/66.1bcaa81c.js"><link rel="prefetch" href="/docs/assets/js/67.326bdf9b.js"><link rel="prefetch" href="/docs/assets/js/68.2b3b63b2.js"><link rel="prefetch" href="/docs/assets/js/69.c9b1a1a9.js"><link rel="prefetch" href="/docs/assets/js/70.a5fede78.js"><link rel="prefetch" href="/docs/assets/js/71.288d9643.js"><link rel="prefetch" href="/docs/assets/js/73.47155429.js"><link rel="prefetch" href="/docs/assets/js/74.4d4738f8.js"><link rel="prefetch" href="/docs/assets/js/75.659a325f.js"><link rel="prefetch" href="/docs/assets/js/76.a263f692.js"><link rel="prefetch" href="/docs/assets/js/77.9d86d59e.js"><link rel="prefetch" href="/docs/assets/js/78.a41b5d88.js"><link rel="prefetch" href="/docs/assets/js/79.5d0fc4cc.js"><link rel="prefetch" href="/docs/assets/js/8.80d0c81d.js"><link rel="prefetch" href="/docs/assets/js/80.01a11f6a.js"><link rel="prefetch" href="/docs/assets/js/81.9c0207d9.js"><link rel="prefetch" href="/docs/assets/js/82.12be8e04.js"><link rel="prefetch" href="/docs/assets/js/83.00fa5f46.js"><link rel="prefetch" href="/docs/assets/js/84.f14cd53c.js"><link rel="prefetch" href="/docs/assets/js/85.e9d7c0d1.js"><link rel="prefetch" href="/docs/assets/js/86.eb78fbbf.js"><link rel="prefetch" href="/docs/assets/js/87.8a31f5ae.js"><link rel="prefetch" href="/docs/assets/js/88.0f10e6dc.js"><link rel="prefetch" href="/docs/assets/js/89.a17253c9.js"><link rel="prefetch" href="/docs/assets/js/9.2fa568cb.js"><link rel="prefetch" href="/docs/assets/js/90.e56bc609.js"><link rel="prefetch" href="/docs/assets/js/91.5277db18.js"><link rel="prefetch" href="/docs/assets/js/92.b0bb8aa7.js"><link rel="prefetch" href="/docs/assets/js/93.cd790b77.js"><link rel="prefetch" href="/docs/assets/js/94.76bbe227.js"><link rel="prefetch" href="/docs/assets/js/95.4f07ca32.js"><link rel="prefetch" href="/docs/assets/js/96.2b1e8485.js"><link rel="prefetch" href="/docs/assets/js/97.e93b3461.js"><link rel="prefetch" href="/docs/assets/js/98.658063a1.js"><link rel="prefetch" href="/docs/assets/js/99.a3642e6d.js">
    <link rel="stylesheet" href="/docs/assets/css/0.styles.efa081cd.css">
  </head>
  <body>
    <div id="app" data-server-rendered="true"><div class="theme-container"><header class="navbar"><div class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/docs/" class="home-link router-link-active"><!----> <span class="site-name">Guru Note</span></a> <div class="links"><div class="search-box"><input aria-label="Search" autocomplete="off" spellcheck="false" value=""> <!----> <svg aria-hidden="true" class="icon search-icon"><use xlink:href="#icon-search"></use></svg></div> <nav class="nav-links can-hide"><div class="nav-item"><a href="/docs/layout/" class="nav-link">
  CSS 手册
</a></div><div class="nav-item"><a href="/docs/js/" class="nav-link router-link-active">
  JavaScript
</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端进阶" class="dropdown-title"><span class="title">前端进阶</span> <span class="arrow down"></span></button> <button type="button" aria-label="前端进阶" class="mobile-dropdown-title"><span class="title">前端进阶</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>
          前端框架
        </h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/docs/vue/" class="nav-link">
  Vue.js
</a></li><li class="dropdown-subitem"><a href="/docs/react/" class="nav-link">
  React.js
</a></li></ul></li><li class="dropdown-item"><h4>
          拓展语言
        </h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/docs/node/" class="nav-link">
  Node.js
</a></li><li class="dropdown-subitem"><a href="/docs/ts/" class="nav-link">
  TypeScript
</a></li></ul></li></ul></div></div><div class="nav-item"><a href="/docs/devbook/" class="nav-link">
  开发手册
</a></div><div class="nav-item"><a href="/docs/exam/" class="nav-link">
  金九银十
</a></div><div class="nav-item"><a href="/docs/message/" class="nav-link">
  留言板
</a></div> <!----></nav></div></header> <div class="sidebar-mask"></div> <aside class="sidebar"><div class="el-scrollbar"><div class="scrollbar-wrapper el-scrollbar__wrap el-scrollbar__wrap--hidden-default"><div class="el-scrollbar__view"><nav class="nav-links"><div class="nav-item"><a href="/docs/layout/" class="nav-link">
  CSS 手册
</a></div><div class="nav-item"><a href="/docs/js/" class="nav-link router-link-active">
  JavaScript
</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端进阶" class="dropdown-title"><span class="title">前端进阶</span> <span class="arrow down"></span></button> <button type="button" aria-label="前端进阶" class="mobile-dropdown-title"><span class="title">前端进阶</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>
          前端框架
        </h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/docs/vue/" class="nav-link">
  Vue.js
</a></li><li class="dropdown-subitem"><a href="/docs/react/" class="nav-link">
  React.js
</a></li></ul></li><li class="dropdown-item"><h4>
          拓展语言
        </h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/docs/node/" class="nav-link">
  Node.js
</a></li><li class="dropdown-subitem"><a href="/docs/ts/" class="nav-link">
  TypeScript
</a></li></ul></li></ul></div></div><div class="nav-item"><a href="/docs/devbook/" class="nav-link">
  开发手册
</a></div><div class="nav-item"><a href="/docs/exam/" class="nav-link">
  金九银十
</a></div><div class="nav-item"><a href="/docs/message/" class="nav-link">
  留言板
</a></div> <!----></nav>  <ul class="sidebar-links"><li><a href="/docs/js/" aria-current="page" class="sidebar-link">介绍</a></li><li><section class="sidebar-group depth-0"><p class="sidebar-heading"><span>JavaScript</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/docs/js/base/type-of-data.html" class="sidebar-link">JS 的数据类型</a></li><li><a href="/docs/js/base/object.html" class="sidebar-link">JS 对象的常用属性</a></li><li><a href="/docs/js/base/function.html" class="sidebar-link">函数 &amp; 原型链</a></li><li><a href="/docs/js/base/dom-api.html" class="sidebar-link">DOM API 总结</a></li><li><a href="/docs/js/base/node-api.html" class="sidebar-link">节点 API 总结</a></li><li><a href="/docs/js/base/node-list.html" class="sidebar-link">节点中的伪数组</a></li><li><a href="/docs/js/base/dom-operate-across-threads.html" class="sidebar-link">DOM 操作跨线程</a></li><li><a href="/docs/js/base/create-jquery.html" class="sidebar-link">jQuery 中的链式操作</a></li><li><a href="/docs/js/base/plugin.html" class="sidebar-link">JavaScript 插件 / 库</a></li></ul></section></li><li><section class="sidebar-group depth-0"><p class="sidebar-heading open"><span>ES6</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/docs/js/es6/scope.html" aria-current="page" class="active sidebar-link">作用域</a></li><li><a href="/docs/js/es6/new-object.html" class="sidebar-link">新版对象</a></li><li><a href="/docs/js/es6/anatomy-assignment.html" class="sidebar-link">参数处理 &amp; 解构赋值</a></li><li><a href="/docs/js/es6/new-string.html" class="sidebar-link">新版字符串</a></li><li><a href="/docs/js/es6/module-use.html" class="sidebar-link">ES6 模块化</a></li><li><a href="/docs/js/es6/promise-usage.html" class="sidebar-link">Promise</a></li><li><a href="/docs/js/es6/await-and-async.html" class="sidebar-link">async 和 await 的用法</a></li><li><a href="/docs/js/es6/symbol-iterate-generate.html" class="sidebar-link">Symbol &amp; 迭代器 &amp; 生成器</a></li><li><a href="/docs/js/es6/arrow-function.html" class="sidebar-link">箭头函数</a></li><li><a href="/docs/js/es6/new-api.html" class="sidebar-link">ES6 新增 API</a></li></ul></section></li><li><section class="sidebar-group depth-0"><p class="sidebar-heading"><span>JavaScript 进阶</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/docs/js/part/ajax-use.html" class="sidebar-link">AJAX</a></li><li><a href="/docs/js/part/cross-domain.html" class="sidebar-link">跨域 &amp; CORS &amp; JSONP</a></li><li><a href="/docs/js/part/browser-storage.html" class="sidebar-link">浏览器模型</a></li></ul></section></li><li><section class="sidebar-group depth-0"><p class="sidebar-heading"><span>JS 小记</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/docs/js/note/project.html" class="sidebar-link">JavaScript 搭建项目</a></li><li><a href="/docs/js/note/js-garbage.html" class="sidebar-link">JS 的缺陷</a></li><li><a href="/docs/js/note/setInterval-a-setTimeout.html" class="sidebar-link">使用 setTimeout 实现 setInterval</a></li><li><a href="/docs/js/note/array-deduplica.html" class="sidebar-link">数组去重</a></li><li><a href="/docs/js/note/download-file.html" class="sidebar-link">使用 JS 下载文件</a></li><li><a href="/docs/js/note/top-nav.html" class="sidebar-link">设置导航动态显示 / 隐藏</a></li><li><a href="/docs/js/note/anchor-transition.html" class="sidebar-link">JS 实现点击锚点过渡效果</a></li><li><a href="/docs/js/note/data-list.html" class="sidebar-link">数据处理</a></li></ul></section></li></ul> </div></div><div class="el-scrollbar__bar is-horizontal"><div class="el-scrollbar__thumb" style="width:0;transform:translateX(0%);ms-transform:translateX(0%);webkit-transform:translateX(0%);"></div></div><div class="el-scrollbar__bar is-vertical"><div class="el-scrollbar__thumb" style="height:0;transform:translateY(0%);ms-transform:translateY(0%);webkit-transform:translateY(0%);"></div></div></div></aside> <main class="page"> <div class="re-page-top"><h1 class="title"><span>作用域</span></h1> <div class="re-page-index"><div class="page-index-title"><span class="open-catalog"><svg aria-hidden="true" class="icon arrow-right"><use xlink:href="#icon-bold-right"></use></svg> <span class="text">目录</span></span></div> <div class="page-index-content"><div class="page-catalog" style="display:none;"><div class="synopsis-wrap"><ul class="synopsis-ul"><li class="synopsis-li"><a href="/docs/js/es6/scope.html#大纲" class="synopsis-link">大纲</a><ul class="synopsis-ul"><li class="synopsis-li"><a href="/docs/js/es6/scope.html#let-的特点" class="synopsis-link">let 的特点</a></li><li class="synopsis-li"><a href="/docs/js/es6/scope.html#const-的特点" class="synopsis-link">const 的特点</a></li></ul></li><li class="synopsis-li"><a href="/docs/js/es6/scope.html#变量-let" class="synopsis-link">变量 let</a><ul class="synopsis-ul"><li class="synopsis-li"><a href="/docs/js/es6/scope.html#let-语法的由来-相较于var的优点" class="synopsis-link">let 语法的由来 (相较于var的优点)</a></li><li class="synopsis-li"><a href="/docs/js/es6/scope.html#let-作用域" class="synopsis-link">let 作用域</a></li><li class="synopsis-li"><a href="/docs/js/es6/scope.html#for-循环中的-let" class="synopsis-link">for 循环中的 let</a></li></ul></li><li class="synopsis-li"><a href="/docs/js/es6/scope.html#常量-const" class="synopsis-link">常量 const</a></li><li class="synopsis-li"><a href="/docs/js/es6/scope.html#相关面试题" class="synopsis-link">相关面试题</a><ul class="synopsis-ul"><li class="synopsis-li"><a href="/docs/js/es6/scope.html#for循环中的-let-声明-i" class="synopsis-link">for循环中的 let 声明 i</a></li></ul></li></ul></div></div></div></div></div> <div class="theme-default-content content__default"><h2 id="大纲"><a href="#大纲" class="header-anchor">#</a> 大纲</h2> <h3 id="let-的特点"><a href="#let-的特点" class="header-anchor">#</a> let 的特点</h3> <ol><li>let 的作用域在最近的<code>{}</code>之间</li> <li>let 语法不允许在 let 声明该变量之前使用该变量</li> <li>let 语法不允许重复声明一个相同的 let 变量</li> <li>for 循环中的 <code>let = i</code> 作用域不会跑出循环</li></ol> <h3 id="const-的特点"><a href="#const-的特点" class="header-anchor">#</a> const 的特点</h3> <ul><li>const 的其他特性与let相同</li> <li>const 只有一次赋值机会，并且声明时必须赋值</li></ul> <h2 id="变量-let"><a href="#变量-let" class="header-anchor">#</a> 变量 let</h2> <h3 id="let-语法的由来-相较于var的优点"><a href="#let-语法的由来-相较于var的优点" class="header-anchor">#</a> let 语法的由来 (相较于var的优点)</h3> <p><strong>声明变量的几种方式</strong></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// ES3 语法</span>
a <span class="token operator">=</span> <span class="token number">1</span>  <span class="token comment">//</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span>
<span class="token comment">// ES6语法</span>
<span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token number">1</span>
</code></pre></div><p><strong>var 变量提升</strong></p> <p>理论上下面的代码应该报错，并且不会a被声明</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
        <span class="token keyword">var</span> a 
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>实际上上面的代码是这样执行的</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">var</span> a
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><blockquote><p>var 变量提升只有在遇到函数时才会停止，其他作用域都无法阻止 var 的变量提升</p></blockquote> <p><strong>如何限制变量作用域</strong></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// ES6之前，使用立即执行函数</span>
<span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span>
    window<span class="token punctuation">.</span><span class="token function-variable function">sun</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>

<span class="token comment">// ES6 语法，只需要在 {} 中使用let声明</span>
<span class="token punctuation">{</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
    window<span class="token punctuation">.</span><span class="token function-variable function">sun</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h3 id="let-作用域"><a href="#let-作用域" class="header-anchor">#</a> let 作用域</h3> <p>let 声明的变量，作用域只在<code>{}</code>中有效，例如：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 报错，找不到变量 a</span>
</code></pre></div><p>let 变量在声明之前无法使用（这个现象叫：Temp Dead Zone [临时死区] 就是在 let 声明之前的区域）</p> <p>例1：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
    <span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 报错，无法在 let 声明之前使用该变量</span>
        <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">2</span>
        <span class="token punctuation">{</span>
            <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">3</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>例2：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">if</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 报错，变量 a 不存在</span>
</code></pre></div><p>let 变量无法重复声明，例如：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">2</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 报错，无法重复声明变量</span>
<span class="token punctuation">}</span>
</code></pre></div><p>但 let 可以重新赋值</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
    <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span>
    a <span class="token operator">=</span> <span class="token number">2</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 输出：2</span>
<span class="token punctuation">}</span>
</code></pre></div><h3 id="for-循环中的-let"><a href="#for-循环中的-let" class="header-anchor">#</a> for 循环中的 let</h3> <p>在 for 循环的括号中使用 let 声明的i也无法被 for 循环外部使用</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">5</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span> <span class="token comment">// 打出 0 1 2 3 4</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre></div><h2 id="常量-const"><a href="#常量-const" class="header-anchor">#</a> 常量 const</h2> <p>const 声明一个常量，只允许赋值一次，无法修改。并且 const 声明必须赋值。</p> <p>const 的其他特性与 let 都相同</p> <p>例1</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
    <span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token number">1</span>
    a <span class="token operator">=</span> <span class="token number">2</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 报错，const 只有一次赋值机会</span>
<span class="token punctuation">}</span>
</code></pre></div><p>例2</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">{</span>
    <span class="token keyword">const</span> a <span class="token comment">// 报错，const 声明必须赋值</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="相关面试题"><a href="#相关面试题" class="header-anchor">#</a> 相关面试题</h2> <p><strong>1. 使用var声明一个变量 a，并添加一个函数，在函数中打出 a，如果在函数调用之前有一行未知代码，那么变量 a 的值有可能会受未知代码影响吗？</strong></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span>
<span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 1</span>
<span class="token punctuation">}</span>
xxx <span class="token comment">/* 这里有一行未知代码 */</span>
<span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre></div><p>假如在未知代码中重新赋值一个 a 变量，a 的值就会改变</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span>
<span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// 1</span>
<span class="token punctuation">}</span>
a <span class="token operator">=</span> <span class="token number">3</span> <span class="token comment">/* 未知代码 */</span>
<span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 输出：3</span>
</code></pre></div><p><strong>2. for循环中使用var声明的变量 i 最终的值是多少</strong></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">6</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
</code></pre></div><p>答案：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> i
<span class="token keyword">for</span><span class="token punctuation">(</span>i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">6</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token comment">// 0,1,2,3,4,5</span>
<span class="token punctuation">}</span>
<span class="token comment">// 当 i 等于 6 时，跳出循环，输出 i</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
</code></pre></div><p><strong>3. 在 for 循环中存放一个函数，函数的内容是打出 i 的值。存放函数后调用.onclick方法执行该函数，那么该函数最后打出的结果是什么</strong></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> i
<span class="token keyword">for</span><span class="token punctuation">(</span>i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">6</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'函数执行的i'</span><span class="token punctuation">)</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
    button<span class="token punctuation">.</span>onclick <span class="token operator">=</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token comment">// 点击事件调用该方法</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'最后输出的i'</span><span class="token punctuation">)</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
<span class="token comment">// 最后打出的 i 是多少</span>
</code></pre></div><p>答案：</p> <p>最终 i 会打出 6，因为 onclick 事件是在代码执行完成后执行的</p> <p>JS Bin链接：https://jsbin.com/noselux/2/edit?html,js,output</p> <h3 id="for循环中的-let-声明-i"><a href="#for循环中的-let-声明-i" class="header-anchor">#</a> for循环中的 let 声明 i</h3> <p>onclick点击与循环结果：</p> <p>如下代码点击 li 列表中的每一项打出的结果应该是多少</p> <p>html</p> <div class="language-html extra-class"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>ul</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span>内容1<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span>内容2<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span>内容3<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span>内容4<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span>内容5<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span><span class="token punctuation">&gt;</span></span>内容6<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>ul</span><span class="token punctuation">&gt;</span></span>
</code></pre></div><p>JS</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> liList <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'li'</span><span class="token punctuation">)</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> liList<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    liList<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span> <span class="token comment">// 输出 6</span>
</code></pre></div><p>上面的代码输出的是 6，并且不论点击哪个 li 元素都会输出 6，这是因为onclick事件是在for循环执行完毕后才能触发的，因为只有在代码运行完成时才会显示页面，为了解决这个问题，可以：</p> <p>修改其中的JS代码，将 i 改为用let声明的新变量 j (不能使用var声明，var声明变量会提升)</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> liList <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'li'</span><span class="token punctuation">)</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> liList<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> j <span class="token operator">=</span> i
    liList<span class="token punctuation">[</span>j<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>j<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>此时点击每个列表将显示对应的 i 的值</p> <p>JS Bin链接：https://jsbin.com/nawiqun/3/edit?html,js,output</p> <p>上面的代码可以简化为：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> liList <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'li'</span><span class="token punctuation">)</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> liList<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    liList<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>其中 <code>i</code> 的作用域就在 <code>for</code> 后面的 <code>()</code> 中，不会跑出去。但 for 循环的 <code>{}</code> 中每次循环都会自动声明一个一模一样的 <code>i</code> ，将 <code>()</code>中的 <code>i</code> 赋给 它。就像下面的代码，假如 <code>()</code> 中的 <code>i</code> 为 <code>_i</code></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> liList <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'li'</span><span class="token punctuation">)</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> _i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> _i <span class="token operator">&lt;</span> liList<span class="token punctuation">.</span>length<span class="token punctuation">;</span> _i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">/* 此处声明了 i0 i1 i2 i3 i4 i5 五个 i */</span>
    <span class="token keyword">let</span> i <span class="token operator">=</span> _i <span class="token comment">// 表示块级&quot;{}&quot;的i = 圆括号&quot;()&quot;中的 i 的值</span>
    liList<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>JS Bin链接：https://jsbin.com/nawiqun/4/edit?html,js,output</p> <p><strong>ES6之前的写法</strong></p> <p>JS部分</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> liList <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'li'</span><span class="token punctuation">)</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> liList<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 在立即执行函数中传入 i 并将 j 声明在函数中，即可保证 j 不会变量提升</span>
    <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">var</span> j <span class="token operator">=</span> arguments<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span>
        liList<span class="token punctuation">[</span>j<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function-variable function">onclick</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>j<span class="token punctuation">)</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre></div></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">上次更新：</span><span class="time">2021-02-17 20:44</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev"><svg aria-hidden="true" class="icon"><use xlink:href="#icon-left"></use></svg> <a href="/docs/js/base/plugin.html" class="prev">JavaScript 插件 / 库</a></span> <span class="next"><a href="/docs/js/es6/new-object.html">新版对象</a> <svg aria-hidden="true" class="icon"><use xlink:href="#icon-right"></use></svg></span></p></div> </main></div><div class="global-ui"><!----><!----></div></div>
    <script src="/docs/assets/js/app.cdec4db9.js" defer></script><script src="/docs/assets/js/2.993bd611.js" defer></script><script src="/docs/assets/js/72.8665d6b8.js" defer></script><script src="/docs/assets/js/7.5a12abef.js" defer></script>
  </body>
</html>
